Providers will be able to see the working line and know that it is Sky and will not provide BB on a Sky line. There systems will NOT be able to see the other line. You will need to get that provided as a new line with Phone services on then add the BB on top.
Either you or your wife can do this but some (BB) providers do not like to provide a 2nd line. You should be able to order a NEW voice line with any provider but you need to be clear that this is a new provide. This may cost you the price of a new line as the 'dormant' one is not working anymore.